Funding: This
work was supported by grant from the Hong Kong Polytechnic University (RPFG).

Competing
interests:The authors have declared that no
competing interests exist.

Conflict
of interest: None.

Copyright:2014 By
the Editorial Department of Journal of Nasopharyngeal Carcinoma. This is an
open-access article distributed under the terms of the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in
any medium, provided the original author and source are credited.

Abstract: Radiation
therapy (RT) is the primary treatment for many head and neck cancers including
nasopharyngeal carcinoma (NPC). While prognosis has been greatly improved with
the advancement of RT technique, radiation-induced complications especially
normal tissue surrounding tumour volume is unavoidable. Genetic factors are
thought to be the most important factors contributing to individual variation
in radiation sensitivity. Over 120 studies have been published since year 2000
to investigate the association of genetic variants to radiation-induced
toxicities in various types of cancer. Candidate gene approach is the most
commonly used approach in published studies, including studies in patients with
NPC. Skin and mucosal toxicities are two of the most common radiation induced
complications in the radiotherapy of NPC patients. However, studies focused on
radiation toxicity in NPC patients are limited. Published literatures focused
on genetic variations and radiation sensitivity in NPC patients are summarized
in this review, and recommendations for future studies are also suggested.

Nasopharyngeal carcinoma (NPC), an endemic disease in
Southern China including Hong Kong, is primarily treated by radiotherapy (RT)
due to its deep-seated anatomical location and its relatively high sensitivity
to radiation, particularly for thehighest incidence subtype, undifferentiated
carcinoma [1]. RT has
evolved from two-dimensional conventional RT techniques to three-dimensional intensity-modulated radiation
therapy (IMRT) techniques to increase dose conformity to target volume [2]. Overall
survival rate and local regional control rate for NPC, especially for advanced
T3-4 diseases, have been improved from 50-75% to about 90% with advanced RT
technique [3].
Chemotherapy is used concurrently with RT to increase local regional control
rate for advance stage disease and to reduce rate of distant metastasis,
although the use of adjuvant chemotherapy is still under debate [3]. Since
the median age of NPC patients is around 50s, quality of life in these patients
is one of the major concerns in providing better patient care. Acute and late
effects of radiation induced toxicities have been receiving growing interest in
the field of radiation oncology, since radiation induced toxicities are
unavoidable and it is difficult to identify which patients are with higher risk
of developing severe radiation induced toxicities before the start of RT. Acute
reactions such as dermatitis, dysphagia, and mucositis and late complications
such as xerostomia and neck fibrosis are some of the most common radiation
induced toxicities regardless of any RT techniques in NPC patients. Dose
escalation to target volume can be achieved by IMRT without compromising
critical organs at risk (OAR) [4]. While
incidence of radiation induced toxicities such as xerostomia, temporal lobe
neuropathy and cranial nerve palsy have been greatly reduced thanks to the
improvement of dose-sparing technique achieved by IMRT [5,6], skin
and mucosal toxicities are unavoidable and remain the major treatment-related
complications that may interrupt treatment schedule. Around 48% and 16% of NPC
patients treated by RT alone suffered from grade 3 or above mucositis and skin
reactions respectively [7]. With
the use of chemotherapeutic agents such as cisplatin, the incidence of
ototoxicity is significantly higher in patients treated with concurrent
chemotherapy and RT (CRT) [8]. Over
61% of NPC patients treated with CRT suffered from grade 3 or above acute
mucositis [8]. The
overall incidence of acute toxicity in any aspect was 83.2% and 53% for
patients treated with CRT and RT respectively [8]. In
terms of late toxicity, about 5% of NPC patients suffered from neck fibrosis
treated with RT alone or CRT [8].
Treatment of radiation induced toxicities is mainly palliative since there is
no effective way to prevent the occurrence of radiation induced toxicities.
Palifermin, a recombinant truncated human keratinocyte growth factor (KGF)
protein, has been showed to reduce the incidence of severe oral mucositis in
two clinical trials with head and neck cancer patients. Management of acute
skin reactions include symptom-relieving agents such as moisturizing cream and
hydrocortisone creams while anti-inflammatory and antioxidant treatments are
used for late fibrosis [9,10].

Patients
receiving similar treatment modality have been found to experience different
levels of radiation induced toxicities. There are probably several risk factors
contributing to the individual variations in normal tissue sensitivity to
radiation. One of the risk factors in increasing the risk of radiation induced
complications is treatment-related. The correlation between total irradiated
volume and complication risk has been reported in many cancers [11]. For
breast cancer patients with larger breast volume, higher risk of suffering
severe skin reactions has been found. Correlation of irradiated volume in lung
cancer and liver cancer patients and increased risk of radiation induced
toxicities have also been reported [11]. Use of
RT techniques and chemotherapeutic agents, in addition to total radiation dose
and number of fractions, may contribute to the increase of developing radiation
induced complications [9,11].
Patient’s clinical characteristic is another important confounding factor
that increased risk of developing radiation induced complications. Age, gender,
medical complications and lifestyles such as smoking and drinking habits are
patient-related risk factors [11].
However, up to 80% of the radiation induced toxicities cannot be explained by
these known factors [12]. Genetic
factors seem to be a most important underlying factors contributing to
variation in radiosensitivity in normal tissue. Several genes have been
identified throughout the past years that are related to increase
radiosensitivity. Patients with genetic disorders such as Ataxia-telangiectasia
and Fanconi’s anemia are known to be hypersensitivity to radiation due to
truncated mutations in genes responsible for cell cycle regulation and DNA
repair pathways, such as ataxia-telangiectasia mutated (ATM), DNA ligase IV (LIG4),
and genes belong to Fanconi anaemia complementation group (FANC) family [12-15]. RT
induces cell death mainly through the generation of reactive free radicals that
interact with DNA, RNA, proteins, and plasma membrane [16]. Many signal
transduction pathways are activated by DNA double-strand break (DSB) to repair
DNA damages, since DSB is the primary lethal cell damage during RT [17]. ATM
protein recognizes the complexes formed by DSBs and DNA repair proteins and the
activity of ATM further activates other proteins such as transcription factors
p53 [17]. Genes
that involved in cell cycle checkpoint, DNA repair, and removal of reactive
free radicals, such as nuclear factor kappa-B (NF-κB), superoxide
dismutase (SOD), transforming growth factor beta (TGFβ), p53, tumour
necrosis factor (TNF-α), and x-ray repair cross-complementing protein
1 (XRCC1) are activated by the DNA damages [16,18]. As a
result, many genes that are associated with cell cycle regulation and DNA
repair and their association to radiation induced complications have been
investigated using candidate gene approach. Candidate gene approach and
genome-wide association study (GWAS) are the two approaches used in genetic
association studies. Candidate gene approach is a hypothesis-driven approach
that is useful in studying single nucleotide polymorphisms (SNPs) and other
types of genetic variants in genes with known biological roles in disease or
phenotype of interest [19]. In
contrast, GWAS is a hypothesis-free approach that prior knowledge of functional
roles of SNPs in phenotype of interest is not required [20]. A dense
set of SNPs is captured by GWAS comprehensively and unbiasedly. Several susceptibility
loci in complex diseases such as diabetes have been identified in GWAS [20].

3.Genetic Association Studies of Radiation Induced
Toxicities in NPC

A
literature search was performed on PubMed using a combination of keywords
“radiotherapy or radiation therapy”, “radiation induced or
radiosensitivit or hypersensitivit or radiotoxicit or normal tissue toxicity or
complication”, and “polymorphism or single nucleotide polymorphism
or SNP or genetic variant” (as of December 14, 2013). Over 120 published
original and review articles were identified through literature search, manual
search of citations from identified articles and selected journals (Figure 1).
Candidate gene approach is the most commonly used in identified articles. Among
these articles, only 4 articles included NPC patients, and 3 out of 4 articles
were published by the same research group [21-24]. A
summary characteristic of studies included NPC patients and data from our group
(unpublished data) is shown is Table 1. Positive findings from included studies
are summarized in Table 2. With the low incidence of NPC worldwide except
endemic areas [1], it is
not surprising that there is a lack of information on genetic association study
on radiation induced toxicities in NPC patients Even with the inclusion of
studies focused on radiation induced toxicities in head and neck cancers [25-28], the
total number of studies is still limited (data not shown). All five studies are
retrospective, case-control studies with different classification of
“normal” and “severe” toxicity. Among genetic
association studies included NPC patients, skin reactions and mucositis are the
major toxicities of interest. While genetic association to acute skin reactions
and acute mucositis was evaluated in two studies, the other three studies published
by the same research group focused on subcutaneous and deep tissue fibrosis
only. The largest sample size among the five studies is 155 with median 40
months of follow-up period after RT [21]. Genes
investigated in these studies are involved in cell cycle regulations (ATM,
CDKN1A, HDM2, TP53), inflammatory response (TGFβ1),
DNA repair (LIG4, XRCC1, XRCC3, XRCC4, XRCC5), and endogenous oxidative
stress defense (SOD2). It is interesting to note that genetic variant
rs25487 (Arg399Gln) located in XRCC1 was associated with increased risk
of acute dermatitis and acute mucositis in study by Li et al. while in
study by Alsbeih et al., the minor allele was associated with lower risk
of developing subcutaneous and deep skin fibrosis (Odd ratio = 0.41, CI =
0.21-0.79) [21,22]. Study
by Pratesi et al. included seven types of head and neck cancers also
showed increased risk of rs25487 to acute mucositis (OR = 3.01, CI = 1.27-7.11)
[27].
However, these significant associations were unable to be replicated by our
group with similar sample size (unpublished data). Conflicting results may be
due to variation in control and case classification, treatment modality,
population stratification for studies with Chinese NPC patients, allele
frequency in different ethnic groups, and other confounding factors such as
patient-related clinical information. Effect size of common genetic variants
may be small to moderate that a larger sample size is needed to validate these
positive findings.

Figure 1. Number of
genetic association studies from 2001-2013 (as of December 14, 2013).

*ORs of both
genotype and allele were calculated; only the most significant results were
shown.

4.Reports In Other Cancers

While
there is still lack of available information for NPC-related radiation induced
toxicities, studies of genetic association in other cancers have changed from
candidate gene approach to GWAS to identify genetic variants that are
associated with ethnic group and disease-specific radiation induced toxicities.
In addition, several meta-analysis have been published and addressed on common
radiation induced toxicities arose in different cancers. A validation study
included 1613 breast cancer and prostate cancer patients showed no association
between radiation toxicity and 92 SNPs in 46 genes [29]. A
replication study using breast cancer patients from three independent European
cohorts showed that a SNP located in TNF-α may be associated to radiation
toxicity [30]. Association
of genetic variants located in heat shock protein beta-1 (HSPβ1) to risk of radiation induced
pneumonitis was validated in two independent cohorts of non-small cell lung
cancer patients [31]. The
first GWAS focusing on radiation induced toxicities in African prostate cancer
identified a genetic variant located in follicle-stimulating hormone receptor (FSHR)
that involved in testis development and function is associated with erectile
dysfunction [32]. A
two-stage GWAS performed by the same research group included mainly European
ancestry showed that this SNP may not be the universal biomarker for all ethnic
groups but an ethnic group and prostate cancer specific biomarker [33]. Two
research groups published meta-analysis of genetic variantrs1800469 in TGFβ1
and association to fibrosis in breast cancer patients and mixed cancer patients
[34,35]. Results
from these studies suggested that individual SNP may be ethnic-group and
complication-specific that may not be used as universal biomarker to predict
all types of toxicities induced in different types of cancers.

Conclusions
And Future Directions

While
significant results were reported in many genetic association studies using candidate
gene approach, these results were unable to replicate in subsequent studies
with larger sample size. In addition, results were often conflicting. Future
studies should include large number of samples, prospectively and
retrospectively, in order to perform well-designed study with adequate
statistical power. Two ways to overcome sample size limitation are to perform
meta-analysis and to collaborate with other research groups. One limitation of
meta-analysis is that adequate information may not be able to obtain through
literature search since positive findings are more likely to be published than
negative findings. In order to enhance data pooling for genetic association
studies, a 18-item checklist guideline Strengthening the Reporting Of Genetic Association
studies in Radiogenomics (STROGAR) was suggested by the Radiogenomic consortium
[36,37]. This
guideline was modified based on the STrengthening the REporting of Genetic
Association Studies (STREGA) recommendation [38] to
include information related to RT. Besides meta-analysis, establishing
international collaboration may help to increase the sample size, reduced
confounding factors by standardizing protocols and experimental design, and to
perform population-based analysis. Individual gene expression profiles and
genetic profiles may be combined to investigate individual variation in radiation
induced toxicity more effectively. Genes associated with cell cycle regulation
and apoptosis pathways showed significant changes in expression levels 2 hours
after irradiation [39]. Gene
expression profiles and genetic profiles in future GWAS can be mapped using quantitative levels of expression (eQTLs) mapping [40]. Several
cis-acting regulators involved in complex diseases have been identified
using eQTLs mapping [40]. By identifying patients with higher
susceptibility to radiation, better patient cancer and customization of
treatment protocol could be achieved that will improve not only the quality of
life, but also the treatment efficacy in future.

5.Conflict of interest

None

6.Funding

This work was supported by grant from the
Hong Kong Polytechnic University (RPFG).

Asia Press is a professional Science, Technology and Medicine publisher, who owns rapid publication, Peer-Reviewed, Open Access Journals. Asia Press aims to promote “knowledge sharing”. As you know, the main barrier for free “knowledge sharing” is the cost of publishing and transfer. In order to encourage scholars and scientists to the max, and devote whole power to realize the aim of “knowledge sharing” and the benefit of “all” mankind, Asia Press performs a permanent policy of no charge for publication and access, and always open its door for authors worldwide.